I. Field and Purview of the Invention:
The present invention concerns in particular a one-piece or a modular basal thumb joint implant for the trapeziometacarpal joint, and can also include an extension of the concept of modularity to other digital joints. The implant generally includes head and stem portions, and it has one or more of the following: a generally acute head-stem attachment orientation; a flanged cross-sectional stem profile; an inwardly curved stem; and an eccentric head attachment site for the stem.
II. Prior Art, and Discovered Problems:
Cases of degenerative or post-traumatic arthritis of the trapeziometacarpal joint are known to leave the joint unstable, disfunctional, and painful. In addressing this problem, the Swanson Basal Thumb Joint was developed for use as an adjunct to resection arthroplasty of the joint. The Swanson implant has a generally simple, linearly straight, non-curved stem, which is squarelike in cross-section; when fitted into the intramedullary canal of the first metacarpal bone it is reportedly designed to resist rotation. It also has a convex head, which fits into a jointlike concave surface fashioned in the opposing distal part of the trapezium bone and which reportedly helps restore joint stability and motion. The Swanson joint is a one-piece unit made from unalloyed titanium (ASTM F67). See, the Wright Medical Technology brochure, “Swanson Titanium Basal Thumb Implant.”
Problems with the foregoing, however, have been discovered to include 1) the straight, non-anatomical countour of the stem, which (a) reduces the rotational stability of the implant, and (b) induces outward mechanical leveraging of the prosthetic head and the proclivity for lateral dislocation of the replaced joint; 2) the demonstrated inferior tribological characteristics of an articulating titanium surface; and 3) the one-piece construction or non-modularity of the implant, which precludes an ability to “mix and match” component parts, consequently requiring a redundant and costly on-the-shelf inventory of implants to assure the availability of an implant which will provide an appropriate head-stem combination that will produce a precise, individualized dimensional fit of both the head and the intramedullary stem parts of the prosthesis. Also, 4) the square-like configuration of the stem, which is the stabilizing part of the composite implant, provides a gross misfit with the curved and elliptically rounded inner counter of the normal medullary canal, which, as a consequence, requires excessive resectional depletion of endosteal bone to obtain an adequately intimate bone-stem interface fit to assure the long term stability of the implant.
In the hip joint implant field, it is known to employ certain principles of modularity. Certain appropriately sized heads that mate with the acetabular socket may be interchanged for assembly with certain properly sized stems of the femoral component.